Re: [PATCH v8 39/43] arm64: RME: Provide register list for unfinalized RME RECs

[Gavin Shan]

On 4/16/25 11:42 PM, Steven Price wrote:
> From: Jean-Philippe Brucker <jean-philippe@xxxxxxxxxx>
>
> KVM_GET_REG_LIST should not be called before SVE is finalized. The ioctl
> handler currently returns -EPERM in this case. But because it uses
> kvm_arm_vcpu_is_finalized(), it now also rejects the call for
> unfinalized REC even though finalizing the REC can only be done late,
> after Realm descriptor creation.
>
> Move the check to copy_sve_reg_indices(). One adverse side effect of
> this change is that a KVM_GET_REG_LIST call that only probes for the
> array size will now succeed even if SVE is not finalized, but that seems
> harmless since the following KVM_GET_REG_LIST with the full array will
> fail.
>
> Signed-off-by: Jean-Philippe Brucker <jean-philippe@xxxxxxxxxx>
> Signed-off-by: Steven Price <steven.price@xxxxxxx>
> ---
>   arch/arm64/kvm/arm.c   | 4 ----
>   arch/arm64/kvm/guest.c | 9 +++------
>   2 files changed, 3 insertions(+), 10 deletions(-)

With below comment addressed.

Reviewed-by: Gavin Shan <gshan@xxxxxxxxxx>

> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> index 4780e3af1bb9..eaa60ba6d97b 100644
> --- a/arch/arm64/kvm/arm.c
> +++ b/arch/arm64/kvm/arm.c
> @@ -1832,10 +1832,6 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
>   		if (unlikely(!kvm_vcpu_initialized(vcpu)))
>   			break;
>   
> -		r = -EPERM;
> -		if (!kvm_arm_vcpu_is_finalized(vcpu))
> -			break;
> -
>   		r = -EFAULT;
>   		if (copy_from_user(&reg_list, user_list, sizeof(reg_list)))
>   			break;
> diff --git a/arch/arm64/kvm/guest.c b/arch/arm64/kvm/guest.c
> index dd379aba31bb..1288920fc73d 100644
> --- a/arch/arm64/kvm/guest.c
> +++ b/arch/arm64/kvm/guest.c
> @@ -671,12 +671,9 @@ static unsigned long num_sve_regs(const struct kvm_vcpu *vcpu)
>   {
>   	const unsigned int slices = vcpu_sve_slices(vcpu);
>   
> -	if (!vcpu_has_sve(vcpu))
> +	if (!vcpu_has_sve(vcpu) || !kvm_arm_vcpu_sve_finalized(vcpu))
>   		return 0;
>   
> -	/* Policed by KVM_GET_REG_LIST: */
> -	WARN_ON(!kvm_arm_vcpu_sve_finalized(vcpu));
> -
>   	return slices * (SVE_NUM_PREGS + SVE_NUM_ZREGS + 1 /* FFR */)
>   		+ 1; /* KVM_REG_ARM64_SVE_VLS */
>   }

KVM_REG_ARM64_SVE_VLS is exposed even SVE isn't finalized. See set_sve_vls() where
it's required that SVE isn't finalized, or -EPERM is returned. So this would be
something like below:

	if (!vcpu_has_sve(vcpu))
		return 0;

	if (!kvm_arm_vcpu_sve_finalized(vcpu))
		return 1;	/* KVM_REG_ARM64_SVE_VLS */

	return slices * (SVE_NUM_PREGS + SVE_NUM_ZREGS + 1 /* FFR */)
		+ 1; /* KVM_REG_ARM64_SVE_VLS */

> @@ -692,8 +689,8 @@ static int copy_sve_reg_indices(const struct kvm_vcpu *vcpu,
>   	if (!vcpu_has_sve(vcpu))
>   		return 0;
>   
> -	/* Policed by KVM_GET_REG_LIST: */
> -	WARN_ON(!kvm_arm_vcpu_sve_finalized(vcpu));
> +	if (!kvm_arm_vcpu_sve_finalized(vcpu))
> +		return -EPERM;
>   
>   	/*
>   	 * Enumerate this first, so that userspace can save/restore in

Since KVM_REG_ARM64_SVE_VLS can be exposed before the vCPU is finalized, it'd better to
move the check after the followup block where KVM_REG_ARM64_SVE_VLS index is copied
to user space.

	/*
         * Enumerate this first, so that userspace can save/restore in
         * the order reported by KVM_GET_REG_LIST:
         */
        reg = KVM_REG_ARM64_SVE_VLS;
        if (put_user(reg, uindices++))
                return -EFAULT;
        ++num_regs;

	if (!kvm_arm_vcpu_sve_finalized(vcpu))
		return num_regs;

Thanks,
Gavin